This invention pertains to clear, strong, crosslinked polymers, obtained by the copolymerization of (A) a linear or branched polysiloxane macromer containing at least two terminal or pendant, polymerizable vinyl groups attached to the polysiloxane by way of a urethane, thiourethane, urea or amide moiety; and (B) a vinyl or divinyl monomer or mixture of monomers, said monomers consisting of 85 to 100% of water insoluble monomers, which polymers are useful in the preparation of soft or hard contact lenses, particularly the hard lenses.
Contact lenses fall into two main categories, normally called "hard" and "soft", but which are better distinguished by the mode in which they are fitted to the eye. Hard lenses are rather loosely fitted to facilitate tear-liquid exchange between lens and cornea; they do this by their "rocking-chair" motion, by which they continuously pump tear liquid out from the space between lens and eye surface. Such tear-liquid exchange is the only way by which all important oxygen is supplied to the cornea of the wearers of conventional hard contact lenses made from polymethyl methacrylate (PMMA). Although hard PMMA lenses are, at least initially, uncomfortable and irritating for the wearer because the blinking eyelid hits a hard edge, they are nevertheless popular because they can be made with great precision by lathing and polishing. This is especially important for the correction of astigmatism with lenses of asymmetric design. Another advantage is that they are very easy to keep clean, requiring no sterilization.
Soft contact lenses on the other hand adhere closely the cornea and are therefore much more comfortable for the wearer; since they allow only limited tear liquid exchange they have to possess oxygen permeability high enough to prevent damage to the eye even if they are worn only for several hours. In all commercial soft lenses this O.sub.2 -permeability is a function of their water-content, that is their hydrogel nature. In addition, water acts as a plasticizer, which gives the lens its softness and the necessary hydrophilicity which allows it to swim and rotate on the cornea, rather than stick to it. Sticking to the cornea is the main problem with hydrophobic soft lenses, such as the otherwise very attractive silicone-rubber lens. Although hydrogel soft lenses represent progress in comfort, precision is compromised since some distortions due to water swelling are unavoidable; because they tend to rotate on the eye, astigmatism is also a much more difficult problem to solve. In addition, protein adsorption and disinfection are major concerns and the gain in comfort is to a great extent balanced by the greater inconvenience of more sophisticated lens-care procedures.
The next major development in contact-lens technology was the manufacture of lenses to be worn continuously, day and night, for up to several weeks. Oxygen permeability of the lens has to be increased manyfold for this purpose since in the closed-eye condition during sleep all the oxygen is supplied to the cornea from the blood-transfused eyelid. Among the several approaches which have not succeeded satisfactorily in this respect are: (1) high water-content hydrogels with up to 70% water; their drawback is mechanical weakness; because of this they have to be made thicker and O.sub.2 -permeability is accordingly reduced; and (2) silicone rubber lenses, which have consistently failed because of their extreme hydrophobicity; they adhere like a suction cup to the cornea and have led to serious eye damage. Attempts to make the surface of silicone-rubber lenses hydrophilic enough to prevent this have not been overly successful, mostly because the treated surface region is too thin to be permanent. For instance, the lenses produced by DOW CORNING under the Trade name SILCON and SILSOFT are treated to form a --Si--OH surface layer which is, however, so thin that it is easily abraded, especially in a hard lens. It is an embodiment of this invention to provide a silicone containing hard lens whose surface is permanently wettable. Another disadvantage of 100% silicone rubber contact lenses is the difficulty with which the edges can be finished and polished and thereby made comfortable to the wearer; this is inherent in all too rubbery materials. It is a further embodiment of this invention to provide silicone containing polymeric materials which range from "soft" to "hard"--using contact-lens terminology--but which are all easily polishable.
Recent examples of silicone containing hydrophobic polymers are described in U.S. Pat. Nos. 4,153,641, 4,189,546 and 4,195,030, and they consist of polymerized or copolymerized high-molecular weight polysiloxane or polyparaffin-siloxane diacrylates and methacrylates. The resulting polymers are hydrophobic. U.S. Pat. No. 4,217,038 describes a surface treatment to render their surface hydrophilic. Other polysiloxanes containing contact lens materials are described in U.S. Pat. Nos. 4,208,362; 4,208,506; 4,254,248; 4,259,467; 4,277,595; 4,260,725 and 4,276,402. All these patents are based on bis-methacrylate esters of linear polysiloxanes-diols. Because of the soft nature of linear polysiloxanes, they are not suited to make the hard, stiff materials necessary for a hard contact lens. U.S. Pat. No. 4,136,250 in addition to the above mentioned bis-methacrylates of linear polysiloxane-diols also discloses bis- and tris-methacrylates of diols and triols in which the functional groups are pendant, not terminal, to the main polysiloxane chain, and in addition compounds which are connected to the polysiloxane through bisurethane linkages. All compositions are, however, hydrogels and as such are only suited for soft contact lenses.
A stiff silicone containing hard lens is described in U.S. Pat. No. 4,152,508 and it consists of a copolymer of a oligosiloxanyl-alkyl acrylate with various comonomers, such as dimethyl itaconate and methyl methacrylate. Although oxygen permeabilities of 3-50.times.10.sup.-10 (cm.sup.3 STP).cm)/cm.sup.2 sec.cm.Hg are claimed, the high amounts of oligosiloxane substituted methacrylate (&gt;40%) necessary to get O.sub.2 -permeabilities higher than 9 make the polymer at the same time too soft to be useful as a hard lens. Only if the Si-monomer constitutes less than 25% of the polymer is polymer hardness sufficiently high for hard contact lens application. Since the siloxane content also affects the wettability adversely, it is, of course, advantageous having to use as little Si in the polymer as possible, just enough to achieve the necessary O.sub.2 -permeability.
Siloxane urethane acrylate compounds which are useful in the preparation of coating compositions for radiation curing are described in U.S. Pat. No. 4,130,708. These siloxane urethane acrylate compounds may be mixed with other ingredients normally found in coating compositions and radiation cured with UV light. The use of such materials for contact lenses is neither described nor suggested by this patent. The coating compositions described in this patent do not include any hydrophilic monomer components.
British Pat. No. 2,067,213 also describes siloxane urethane acrylate compounds useful in the preparation of photocurable coating compositions. The use of such materials for contact lenses is not disclosed. The coating compositions described in this patent do not include any hydrophilic monomer components nor the hydrophobic comonomers described in this invention.
It is known that SiO(CH.sub.3).sub.2 is a more efficient oxygen-transmitter if it is present in form of siloxane polymer than if it is only part of a low molecular weight side group. However, the long polysiloxane chains which are present in compositions of U.S. Pat. No. 4,153,641 and related patents drastically reduce stiffness, leading to rubbery and soft materials. In addition, their compatability with such unrelated polymers as polymethacrylates is poor and phase separation, typical for blends of high-molecular weight polymers, leads to more or less opaque products.
It has now been discovered unexpectedly that if polyfunctional high molecular weight polysiloxanes whose equivalent weight is not greater than 5000 and which are connected to at least two terminal or pendant polymerizable vinyl groups through bis-urethane linkages are incorporated into a crosslinked vinyl copolymer, hard, stiff and clear products are obtained which have excellent O.sub.2 -permeability, up to four times higher than the best prior-art composition, and which are even at a siloxane content of &gt;50% stiff enough to fulfill the requirements of a hard contact lens. It has further, very unexpectedly, been found that the siloxane containing polymers of this invention can be made with better wettability than conventional PMMA hard lenses, despite their high polysiloxane content.
It is contemplated that the unexpected hardness and clarity characteristics of the polymers of this invention are a result of two features: first, in case of a comb-like structure of a high MW polysiloxane, the actual linear chain length of --SiO(CH.sub.3).sub.2 -- between crosslinks is relatively short, despite an overall high molecular weight. This short effective chain length between crosslinks restricts mobility of the polysiloxane chains and minimizes phase separation; at the same time the overall high molecular weight and long continuous strands of --SiO(CH.sub.3).sub.2 -- units and pendant groups insure good oxygen permeability. Secondly, the presence of at least four urethane or urea linkages per mole of polysiloxane contributes through hydrogen-bonding rigidity to the polymer.
The preferred polysiloxane macrometers for synthesis of hard contact-lens materials are ones which contain (a) at least one urethane linkage group per twelve --SiO(CH.sub.3).sub.2 -- units; (b) which contain at least two polymerizable vinyl groups attached pendant to the polysiloxane backbone and therefore contain at least two terminal --Si(CH.sub.3).sub.3 ] units; and (c) contain urethane groups derived from bulky, cycloaliphatic diisocyanates. The preferred comonomers to be combined with the polysiloxane macromers are acrylates and methacrylates which when polymerized by themselves give hard homopolymers with a high glass-transition temperature, such as methyl methacrylate, isopropyl-, isobutyl-, tert.butyl-, cyclohexyl- or isobornyl methacrylate.
Although this invention is primarily directed toward hard contact-lens materials, it was found that clear, strong, but rubbery polymers can also be made. Such polymers, which are useful in a variety of applications like as biocompatible implants, bandages for wound treatment or as soft contact lenses, are thus another embodiment of this invention. It is thus within the scope of this invention to use comonomers which will result in strong, yet flexible and rubbery polymers.